Granulocyte colony-stimulating factor (G-CSF) is the standard growth factor for mobilizing hematopoietic progenitor cells in healthy peripheral blood progenitor cell (PBPC) donors. G-CSF is given daily for 5 days to PBPC donors prior collecting PBPC concentrates by apheresis. PBPC donors given G-CSF experience splenic enlargement and, rarely, spontaneous rupture of the spleen. We evaluated the incidence and time course of splenic enlargement in PBPC concentrate donors and accessed factors effecting size changes. Healthy adults were given G-CSF (10 micrograms/kg/day) for 5 days and a PBPC concentrate was collected by apheresis. Ultrasound was used to assess craniocaudal spleen length prior to giving G-CSF, the day of apheresis, and 4 or 10 days after apheresis. Among 38 healthy subjects a 5-day course of G-CSF causes spleen length to increase in 95% of the donors. The mean spleen length increased from a baseline length of 10.8 ? 1.7 cm to 12.2 ? 1.7 cm and the mean increase in length was 13%. However, spleen enlargement was transient. Four days after apheresis the spleen length fell to 11.3 ? 1.8 cm, but it remained greater than baseline levels. Ten days after apheresis, spleen length fell to 10.5 ? 1.2 cm and did not differ from baseline length. The magnitude of spleen length increase was not affected by donor gender, race, or age. There was no relationship between changes in spleen length and apheresis-day blood counts and chemistries and changes in blood counts and chemistries. We speculate that if G-CSF administration continues beyond 5 days, spleen size will continue to increase placing the donor at greater risk for splenic rupture. A future study will access the kinetics of spleen size changes during a longer 7-day course of G-CSF mobilization.